How Do LED Grow Lights Compare to Traditional Grow Lights?

LED grow lights convert electricity to plant-usable photons at 2.5 to 3.5 µmol/J at the wall. The traditional families they replace sit lower: double-ended HPS runs 1.3 to 1.9 µmol/J, CMH around 1.3 to 1.7, and fluorescent tubes below 1.1. That gap is the headline, and it is not the whole comparison. Two fixtures with the same photon output can still differ in how evenly those photons land, how much heat reaches the leaf, how many service hours the room absorbs, and whether the control system can dim below half power without shifting the spectrum.

This guide compares LED grow lights with traditional grow lights across the four families a licensed cultivation room still encounters: high-pressure sodium, ceramic metal halide, fluorescent tubes, and the incandescent fixtures that predate all of them. It uses system watts at the wall rather than lamp ratings, because that is the number the electricity meter and the HVAC load both see. It also separates the three questions growers usually blend together: cost per photon, cost per gram of flower, and cost per installed fixture. Only the first two belong in a lighting comparison; the third belongs in a finance discussion.

Where the answer depends on your room, the sections below say so instead of averaging it away. A cold-room operation with cheap power and ten-year-old ballasts is not making the same decision as a new build with CO₂ enrichment and a demand charge on the bill.

What Counts as a Traditional Grow Light?

"Traditional" is not one technology. It is four families with different physics, different failure modes and different reasons to still be in a room. Treating them as a single baseline is the first mistake in most comparisons, because a T5 propagation shelf and a 1,000 W double-ended HPS fixture have almost nothing in common beyond the fact that neither is LED.

High-pressure sodium is the benchmark most commercial flower rooms were built around. A 1,000 W lamp produces roughly 1,700 to 2,000 µmol/s, costs little per fixture, and emits a red-heavy spectrum with enough green to penetrate a canopy. Its weaknesses are efficacy, heat, and a lamp that loses output from the first hour. Double-ended fixtures with electronic ballasts are the efficient end of the family; single-ended magnetic systems are the cheap end.

Ceramic metal halide, also sold as LEC or CMH, sits between HPS and LED on spectrum. A 315 W lamp delivers about 500 to 560 µmol/s with more blue than HPS, which keeps internodes shorter and produces tighter vegetative growth. It is common in propagation and mother rooms, and in small flower rooms where spectrum quality matters more than cost per photon.

Fluorescent tubes, mostly T5 HO and T8, remain the default for clones, seedlings and mother plants. Their photon output per watt is low, their useful life is limited by output depreciation, and they are cheap to buy and easy to position close to small plants. A four-lamp T5 fixture draws around 230 W at the wall and produces roughly 200 µmol/s, which is enough for a propagation shelf and not enough for a flowering bed.

Incandescent and halogen fixtures are legacy equipment in the literal sense. They convert most of their input to heat, produce little photosynthetically useful radiation, and survive only as heaters with a light bulb attached. They are included here because grower forums still mention them, not because they belong in a production plan.

Early red-blue LED panels, often called blurple lights, are sometimes filed under "traditional LED" in forum comparisons. They are not a separate physics family. They are LED fixtures with a narrow spectrum and, usually, a low-efficacy generation of diodes. The comparison in this article is between LED as a technology and the discharge and fluorescent families, not between one LED generation and another.

LED vs Traditional Grow Lights: The Main Differences at a Glance

The table below compares the families on the variables that change a purchase decision. System wattage is wall draw per fixture, not the lamp rating. Efficacy is photosynthetic photon efficacy at the wall, so ballast or driver losses are already included. The ranges are planning values across commercially available equipment, not ratings for a specific model.

Two rows in that table explain most of the confusion in the market. The HPS row is not one number: the difference between a magnetic single-ended system and an electronic double-ended system is larger than the difference between some HPS and CMH fixtures. And the LED row spans more than a full µmol/J, which is why a cheap strip and a commercial bar fixture can both be described as "full spectrum LED" while performing like different technologies.

Energy Efficiency: Compare System Watts, Not Lamp Watts

Every lighting quote in horticulture eventually reduces to the same question: how many photons do I get per unit of electricity I pay for? The answer depends on what you count. A 1,000 W HPS lamp does not draw 1,000 W at the wall. The ballast consumes 5 to 15 percent on top of the lamp, depending on type and age. An LED fixture rated at 1,000 W usually quotes input power, so the driver loss is already inside that figure. Comparing a lamp rating to a fixture rating is the most common apples-to-oranges error in a retrofit quote.

The metric that makes the comparison honest is photosynthetic photon efficacy, written µmol/J. It divides the photon output in the 400 to 700 nm band, called PPF and measured in µmol/s, by the electrical input at the wall. The DLC horticultural listing uses the same basis: its current V4.0 requirements set the threshold at 2.5 µmol/J, up from 2.30 in V3.0 and 1.9 in the first 2018 horticultural specification. A fixture below 2.5 is not automatically bad, but it is no longer competitive for a commercial room built today.

Luminous efficacy in lumens per watt is the wrong metric and remains in too many equipment sheets. The lumen is weighted to the human eye, which is most sensitive near 555 nm and nearly blind at 660 nm, where chlorophyll absorbs strongly. A red-heavy fixture can be an excellent plant light and a poor lumen performer. Use lumens to compare two white office fixtures; use µmol/J to compare two grow lights.

The arithmetic then becomes simple enough to check against any quote. To replace the 1,850 µmol/s from the double-ended HPS example, an LED fixture at 2.80 µmol/J needs 661 W. That is 409 W less per fixture, or 38 percent less electricity for the same photon output. Over a 12-hour flowering cycle, 365 days a year, the difference is about 1,790 kWh per fixture annually. At $0.12 per kWh it is roughly $215 per fixture per year, before any rebate and before the HVAC savings that follow the same watt reduction.

Efficacy is not constant across the dimming range, and this matters more for LED than for the older families. Many LED fixtures lose efficiency at the bottom of their dim range, and a few shift spectrum as current drops. A fixture rated at 2.9 µmol/J at full output may deliver 2.6 at 30 percent. Ask for the part-load curve if you plan to run below 50 percent for most of the year, which is exactly what a 1,300 W fixture over a 4 × 6 bed usually does. The guide to µmol/J and how it is measured walks through the measurement tiers and the questions that separate a lab number from a wall number.

Spectrum: A Wider Band Does Not Automatically Mean More Growth

Spectrum is where LED marketing and plant physiology drift apart. A wideband white LED with added red and far-red gives a grower more control over plant shape, stretch and leaf temperature than a fixed HPS spectrum. It does not, by itself, raise yield per photon. The 2021 review by Danziger and Bernstein reached that conclusion after comparing spectrum studies in cannabis: the dominant driver of biomass is the number of photons delivered, with spectrum shaping morphology, secondary metabolism and quality.

The practical differences are still worth understanding. HPS emits most of its radiation between 560 and 700 nm, with a small blue component and little far-red. That spectrum drives flower development efficiently and stretches stems, which is one reason HPS rooms are planned around trellis height. CMH adds blue and fills the green gap, producing shorter internodes and thicker leaves at the cost of efficiency. Fluorescent tubes are blue-rich and weak in red, which suits propagation and poorly suits flowering.

LED can reproduce any of those profiles or combine them. A wideband white plus 660 nm red is the common commercial recipe, with a separate far-red channel on better fixtures. Green light matters more than its absorption spectrum suggests, because it penetrates deeper into the leaf and canopy than red, a point established in leaf-level work by Terashima and colleagues in 2009. In a dense canopy, that penetration reaches chloroplasts that red photons never reach. It is a reason to prefer a white base over a narrow red-blue panel, not a reason to expect a yield increase.

Those shares are approximate and change with lamp age, ballast type and LED binning. They are useful for understanding why a room behaves differently after a conversion, not for predicting a harvest. A room moving from HPS to a wideband LED with the same PPFD usually sees shorter internodes and a denser canopy, which changes airflow, penetration and the defoliation plan. That is a management change, not a spectrum bonus. The guide to spectrum for cannabis flower covers the band-by-band evidence and the trials behind it.

Heat, Leaf Temperature and the HVAC Bill

Every watt that enters a room leaves it as heat, whatever the fixture. The conversion is fixed: 1 W equals 3.41214 BTU per hour. A 1,000 W LED and a 1,000 W HPS add the same heat to the room at the same wall draw, which is why the claim that LEDs produce no heat is wrong in the only sense that matters to a mechanical engineer.

What differs is the path the heat takes. HPS emits a large share of its energy as radiant heat in the beam, so the canopy and the top leaves absorb it directly. Leaf temperature can run 2 to 4 °C above room air under HPS, and the HVAC system sees that radiant load at the canopy rather than at the ceiling. LED fixtures emit little radiant heat toward the plants. Their heat is mostly convective at the fixture body and driver, which means it can be captured and exhausted near the ceiling before it reaches the canopy, but it also means the room air has to run warmer to hold the same leaf temperature.

That second point is the one growers miss after a conversion. If a room moves from HPS to LED and the thermostat stays at the old setpoint, leaf temperature drops, transpiration slows, and the crop looks overwatered or stalled. The fix is usually to raise air temperature by 1 to 3 °C and re-check VPD, not to add heat at the canopy. The VPD guide covers the leaf-to-air relationship and the setpoints that follow a lighting change.

The HVAC comparison is not "LED has less heat". It is "LED puts the same heat in a place that is easier to remove, using fewer watts to deliver the same photons". A room that cuts lighting watts by 38 percent cuts the heat load by 38 percent, and the air-conditioning tonnage follows, but only if the fixtures are laid out so the convective plume does not sit in the canopy. A bar fixture mounted 15 cm above the tops with no extraction path can still cook the top layer.

Lifetime, Depreciation and the Maintenance Line

Lamp life is the traditional argument against HPS, and it is usually overstated in both directions. A modern HPS lamp is rated for 10,000 to 24,000 hours, but commercial flower rooms commonly replace at about 10,000 hours because output falls steadily and the room is not paid to run dim lamps. A 12-hour cycle reaches 10,000 hours in roughly 12 to 14 months. CMH lamps last longer than HPS in hours but lose output faster in the last third of life. Fluorescent tubes are cheap and decline quietly, which is why a propagation shelf can look dim for a season before anyone replaces the lamps.

LED life is quoted in maintenance terms rather than survival. L90 is the hours until output falls to 90 percent of the initial value. Commercial fixtures claim 50,000 to 90,000 hours, and the DLC V4.0 specification requires Q90 of at least 36,000 hours on the 400 to 700 nm band, with drivers and fans rated for at least 50,000 hours at the maximum rated ambient temperature. Q90 counts photons, not lumens, which matters for a plant light. A fixture with no maintenance claim is priced on year one, not on year eight.

The maintenance ledger is where the operating case is won or lost. A 100-fixture HPS room replaces 100 lamps roughly every 13 months, plus ballast failures and the labour to reach every hood. An LED room replaces drivers occasionally and cleans bars. The difference is not only the parts bill; it is the downtime, the access equipment and the risk of a lamp that fails in week six of flower. Budget the labour, not just the lamps.

LED Strips and Bars vs Traditional Fixtures: The Form Factor Difference

A grow light is not only a photon source. It is a shape that decides where the photons land. A single-ended HPS in a hood is a point source with a reflector: intense under the centre, weaker at the edges, and sensitive to mounting height. A bar or strip fixture distributes the same diodes across the footprint, so the distance from emitter to canopy varies less and the light arrives from more angles. That is why two fixtures with identical PPF can produce different PPFD maps.

For energy efficiency specifically, the form factor matters in three ways. First, a bar fixture can sit closer to the canopy without burning the tops, which reduces the wall losses that occur when light spills outside the bed. Second, a distributed source produces a flatter map at the same average PPFD, so you can run a lower average and still meet the weakest plant's requirement. Third, a strip fixture spreads heat across the ceiling instead of concentrating it under a hood, which changes the local temperature gradient and the amount of fan energy needed to mix the room.

The comparison with fluorescent strips is not close on efficiency. A T5 HO lamp at roughly 0.9 µmol/J at the wall delivers about a third of the photons per watt of a mid-tier LED strip at 2.6 to 2.9. The T5 still wins on purchase price for a small propagation shelf, and it still works when the shelf is 20 cm deep and the plants are 10 cm tall. For a commercial propagation room running 18 hours a day, the electricity difference pays for the LED strip within a few cycles.

The comparison with HPS is closer than the efficacy numbers suggest, because a bar fixture changes the room layout. A 4 × 6 bed that ran two 1,000 W HPS hoods can run two 1,000 W LED bars at the same photon output with less overlap and a flatter map, or one 1,300 W bar at part load. The fixture-count guide works through the geometry, including rows, aisles and the overlap you need between fixtures. The short version: buy photon output per bed, not watts per fixture, and check the PPFD map at your mounting height rather than at the height in the brochure.

Dimming and Control: What Each Technology Can Actually Do

Dimming is the quiet advantage of LED in a commercial room. Most HPS systems run on magnetic ballasts that switch on and off, with a few electronic ballasts offering step dimming or a 50 to 110 percent range. CMH dimming exists but is limited and often shortens lamp life. Fluorescent dimming requires compatible ballasts and is rarely installed in a grow room. LED fixtures commonly accept 0 to 10 V, resistance, PWM or a digital protocol, and the better ones hold spectrum and efficacy across most of the range.

That control changes how a room is operated. A propagation area can run at 30 percent, a vegetative room at 60, and a flower room at full output without changing fixtures. A room can dim during a heat event instead of shutting down circuits. A multi-zone room can hold different PPFD targets on the same schedule. None of this is free: dimming below 10 percent may be unstable, some drivers have a minimum load, and a few fixtures shift spectrum as current falls. Ask for the dimming curve, not just the dimming range.

The operational value shows up on the electricity bill in two places. The obvious one is lower kWh when the room runs below full output. The less obvious one is demand: a room that dims during a peak interval can reduce its billed demand where the tariff uses a 15-minute integrated window, which is a different saving from the energy charge. The guide to demand charges and lighting load explains why staggering start times does not do the same thing. Dimming changes power; switching time does not.

Upfront Cost, Payback and Rebates

LED costs more per fixture and less per photon. A 1,000 W bar fixture with a 2.8 µmol/J rating costs several times what a 1,000 W HPS fixture costs, but it also replaces more than one HPS fixture in photon terms and removes the lamp and ballast line from the budget. The comparison that matters is cost per µmol/s of installed output, then cost per kWh saved over the hours the room actually runs.

Three variables move the payback more than the fixture price. Utilization hours decide how fast the electricity saving accumulates: a 12-hour flower room runs 4,380 hours a year, a propagation room at 18 hours runs 6,570. Local electricity price decides what each kWh is worth, and a demand charge can add 20 to 50 percent to the effective cost of running lights at peak. Rebates decide the net capital: DLC-listed fixtures qualify for many utility programmes, and the rebate is usually tied to the listing and the measured efficacy, not to the brand.

The full cash-flow model, including HVAC capital, rebates and the difference between yield per square metre and yield per dollar, is worked through in the LED vs HPS comparison for commercial cannabis. This article deliberately does not repeat those numbers, because the HPS case has its own assumptions and mixing two models in one page is how bad payback claims get made. The short rule: if the room runs more than 4,000 hours a year and power costs more than $0.10 per kWh, the LED case usually closes; if the room runs 2,000 hours a year on cheap hydro, it may not.

Where Traditional Grow Lights Still Win

A comparison that ends with "LED wins everywhere" is not useful, because it is not true. There are rooms where the older technology is the rational choice, and a supplier who cannot name them is not doing the arithmetic.

Cold rooms with a real heating load. In a northern facility, the heat from HPS may replace natural gas or electric resistance heating for part of the year. An LED conversion cuts the lighting bill and the cooling load, then adds a heating bill in winter. The net saving is smaller than the wattage difference suggests, and in a small room with a cheap gas supply it can disappear.

Short-horizon and leased spaces. A three-year lease with no renewal option does not justify a five-year payback. A grower who may move should compare the resale value and the installation cost, not only the operating cost. HPS fixtures are cheap to abandon; LED fixtures are worth moving, which changes the decision.

Existing electrical infrastructure. A room wired for 1,000 W HPS at 240 V may need new circuits, panels or a transformer for a higher-wattage LED layout, even though the LED uses fewer watts overall. If the service is already at capacity, the retrofit can require an electrical upgrade that the operating saving has to absorb.

Propagation and mother rooms at small scale. A T5 shelf is still the cheapest way to light 20 clones. The efficiency gap is real, but the absolute kWh is small, and the capital is better deployed in the flower room first.

Rooms where the spectrum is already working. A cultivar that has been selected and managed under HPS for years may respond differently under a wideband LED: shorter internodes, denser canopy, different dry-down. That is not a reason to stay, but it is a reason to convert one room and compare before converting the site.

A Stage-by-Stage Comparison for Cannabis Rooms

The right fixture depends on the stage, not only on the technology. The table below gives the practical options by stage for a licensed indoor room. PPFD values are targets at canopy height, not fixture ratings, and they assume ambient CO₂ unless stated.

The stage table also explains why a single-fixture room is a compromise. A fixture sized for late flower is overpowered for propagation and cannot dim low enough to be useful; a fixture sized for propagation cannot reach flower DLI. Two smaller fixtures, or one dimmable fixture with a wide range, cover more of the cycle. That is a layout decision, and the PPFD and DLI guide gives the arithmetic for each stage.

How to Compare Two Quotes Without Getting Fooled

Most lighting quotes are not lying. They are answering different questions. A traditional quote lists lamp watts, a fixture count and a price. An LED quote lists input watts, a PPF figure and a footprint. To compare them, put both on the same six lines.

Wall draw per fixture. Ask for input power including driver or ballast. If the supplier quotes "lamp watts", add 5 to 15 percent for the ballast, then compare.

PPF and efficacy at the wall. Ask for PPF in µmol/s and the test basis. Divide PPF by wall watts. A fixture with no PPF figure is being sold on wattage, which is not a photon metric.

A PPFD map at your mounting height. Not the brochure height, not the centre reading. Ask for nine points across the actual bed at the distance you will hang the fixture. Average PPFD and uniformity together decide whether the fixture covers the canopy.

Spectrum with a measured SPD. A spectrum chart drawn by marketing is not data. Ask for the measured SPD and the share of photons in each band if spectrum is part of the decision.

Maintenance claim and warranty. L90 or Q90 hours, the test temperature, and the warranty terms for diodes, driver and labour. A five-year warranty that excludes the driver is a two-year warranty.

Control compatibility. 0 to 10 V, resistance, PWM or digital, and the dim range over which the fixture holds its rating. If you run a controller, ask for the protocol list and the number of fixtures per channel.

Run those six lines for both quotes and most of the ambiguity disappears. What remains is the part no spec sheet covers: the supplier's ability to support the fixture in year three, and whether the PPFD map they sent was measured on the same model they are shipping.

Replacing, Retrofitting or Running Hybrid

There are three ways to move a room from traditional to LED, and they carry different risks.

Full replacement is the cleanest. The room is re-laid out around the new fixture shape, the circuits are re-balanced, and the control system is set up once. It has the highest capital cost and the shortest disruption if the install is planned around a crop break.

Retrofit lamps keep the existing hoods and ballasts and replace the lamp with an LED unit. The saving is real but smaller than a full conversion, because the old reflector and ballast remain, and some ballasts are incompatible with retrofit lamps or void the lamp warranty. A magnetic ballast may not start an LED retrofit reliably; an electronic ballast may, depending on the model. Check the compatibility list before buying a pallet.

Hybrid rooms mix LED bars with HPS, usually during a phased conversion or to keep radiant heat in a cold room. The combination can work, but the control system has to manage two technologies with different dimming behaviour and different spectra. It is a transition state, not a design target.

Whichever route you take, the first 30 days decide whether the conversion sticks. Leaf temperature, VPD, irrigation frequency and canopy density all shift when the fixture changes. The first 30 days after a lighting retrofit covers the measurements to take and the adjustments that prevent a slow start from being blamed on the fixture.

FAQ

The comparison between LED and traditional grow lights comes down to a few measurable lines: wall watts, PPF, PPFD map, maintenance claim and control range. Everything else is a story about a lamp. If a quote cannot supply those five, the number on the box is not yet a decision.